Render separate "back part" and "front part" so I can place something in between in other program?
A common technique in 3D is to render a Z-Depth pass. With this Z depth used as a mask in your favorite compositing app you can place any object at any depth (you threshold the mask to desired value with levels).

The advantage of this is that you have the freedom to change the Z-position of your 2D objects in postproduction. Exporting a Z-Depth pass is also often faster than rendering multiple times.

You can create Z-Depth pass with custom material, mist pass or remapping the Z pass values:

How can I get a Depth of field render pass?

Compositing fog destroys PNG alpha in Images as Planes when using Cycles

Here is how to use such pass in After Effects:

Video Copilot Explosive training (At 35 mins)

He mixes 3D debris and objects with 2D explosions through Z-depth mask.